Leadframe inductors

ABSTRACT

The present invention integrates an inductor into a semiconductor package by integrally forming inductive segments in the leadframe. The inductive segments may be connected directly to a lead of the leadframe, or indirectly to a lead or a bond pad on a semiconductor die via wirebonds to form an inductor. The inductance value for the resultant inductor is typically controlled by the point of contact for the wirebonds or the leads about the inductive segment. The inductance values may also be controlled by the shape and size of the inductive segments. The leadframe may be formed to support multiple inductive segments, and one or more configurations, including those using one or more die flags to support a like number of semiconductor die.

This application is a Continuation of U.S patent application Ser. No.10/456,320 filed Jun. 6, 2003 now U.S. Pat. No. 6,765,284, currentlyallowed, which is a Continuation of U.S. application Ser. No. 10/370,234filed Feb. 20, 2003 and issued as U.S. Pat. No. 6,608,367 on Aug. 19,2003, which is a Divisional of U.S. application Ser. No. 10/082,380filed Feb. 25, 2002 and issued as U.S. Pat. No. 6,621,140 on Sep. 16,2003, the disclosures of which are incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates to inductors, and in particular to forminginductors in leadframes for semiconductor packages.

BACKGROUND OF THE INVENTION

Industry trends in wireless communications are forcing increasedintegration, size reduction, and cost reduction. Many radio frequencycircuits require matching, filtering, and biasing networks, whichrequire inductors having relatively high inductance values with lowloss. In general, the higher the inductance value, the larger and moreexpensive the inductor. Further, the precision of the inductance for theinductor is proportional to its cost. In many applications, inductorscontribute a significant portion of the overall cost of circuitimplementation.

Traditionally, there have been four options available for providinginductance in association with an integrated circuit. The first and mostcommon option is for the end manufacturer to add discrete inductors intheir final assemblies in association with other integrated circuits anddiscrete components. Adding discrete inductors is an unattractive optionfor the end manufacturer due to the physical space required on the finalassembly for the inductor and the cost of the inductor.

A second option is to implement the inductor using wirebonds. Wirebondsare thin wires or ribbons that typically connect portions of asemiconductor die to the leads in the semiconductor package. Whenimplementing an inductor, the wirebonds may be used in traditionalfashion between a bond pad on the semiconductor die and a lead, as wellas between bond pads on the semiconductor die. Unfortunately, wirebondsprovide limited inductance and have proven to be electrically lossy. Athird option is to actually create or place an inductor on thesemiconductor die. Implementing an inductor on a semiconductor die hasproven to be very expensive, electrically lossy, and given the limitedsize of the die, unfeasible in providing higher inductance values.

A fourth option is to design a module package having a substrate onwhich an inductor may be incorporated through surface mount or printedcircuit board fabrication techniques. This option has the samelimitations as having the end manufacturer incorporate the inductor inits final assembly. The result is essentially passing the cost on to themodule fabricator instead of the final assembler.

Accordingly, there is a need for a cost-effective technique forimplementing and integrating inductors into semiconductor packages.There is a further need for these inductors to have sufficientinductance for matching, filtering, and biasing networks in wirelesscommunication applications.

SUMMARY OF THE INVENTION

The present invention integrates an inductor into a semiconductorpackage by integrally forming inductive segments in the leadframe. Theinductive segments may be connected directly to a lead of the leadframe,or indirectly to a lead or a bond pad on a semiconductor die viawirebonds to form an inductor. The inductance value for the resultantinductor is typically controlled by the point of contact for thewirebonds or the leads about the inductive segment. The inductancevalues may also be controlled by the shape and size of the inductivesegments. The leadframe may be formed to support multiple inductivesegments, and one or more configurations, including those using one ormore die flags to support a like number of semiconductor die.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a top view of a traditional leadframe package having anattached semiconductor die according to the prior art.

FIG. 2 is a leadframe constructed according to one embodiment of thepresent invention.

FIG. 3 is the leadframe of FIG. 2 having an attached semiconductor dieand an inductor integrated into the leadframe according to oneembodiment of the present invention.

FIG. 4 is a partial cross-section of a semiconductor having theleadframe illustrated in FIG. 3 and an associated printed circuit boardor mounting substrate.

FIG. 5 is an alternate leadframe configuration according to a secondembodiment of the present invention.

FIG. 6 is an alternate leadframe configuration according to a thirdembodiment of the present invention.

FIG. 7 is an alternate leadframe configuration according to a fourthembodiment of the present invention.

FIG. 8 is another leadframe alternative wherein the leadframe supportstwo semiconductor die.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

With reference to FIG. 1, a configuration for a typical leadframe 10 isillustrated to include multiple leads 12 about the periphery of a dieflag 14. The leads 12 and die flag 14 are generally formed of the samematerial, have the same, uniform thickness throughout the leadframe 10,with the exception of mold locking features, which are less thick, andare electrically isolated from one another. Although FIG. 1 illustratesthe top portion of a leadframe 10, the bottom portion of the leads 12and die flag 14 are left exposed to facilitate contact with electricalcontacts or traces of a substrate or printed circuit board (PCB), whichwill be described in greater detail below. A semiconductor die 16 havingbond pads 18 is placed on the die flag 14. Wirebonds 20 are used toconnect select ones of the bond pads 18 to one or more correspondingleads 12 of the leadframe 10. Notably, the term “wirebond” includestraditional wirebonds, ribbon bonds, and any conductive configurationused to selectively connect the semiconductor die 16 to parts of theleadframe 10.

As noted, the bottom of the leads 12 and die flag 14 generallyfacilitate electrical contact to other circuitry. Typically, thesemiconductor die 16 is attached to the die flag 14 using a conductiveor nonconductive bonding dielectric and any connections from thesemiconductor die 16 to the die flag 14 are facilitated using wirebonds20. In most embodiments, the die flag 14 provides a ground contact,wherein the leads 12 facilitate signal contact to the semiconductor die16 via the wirebonds 20. Notably, the leads 12 and die flag 14 that makeup leadframe 10 of prior art devices are only used for contacts withexternal traces on substrates or PCBs.

Turning now to FIG. 2, a leadframe 10 according to one embodiment of thepresent invention is illustrated. The leadframe 10 includes leads 12 anda die flag 14, as well as inductive segments 22, which are part of theleadframe and are capable of being used to form inductors. Asillustrated, the three inductive segments 22 are parallel to one anotherand run between respectively opposing leads 12. Preferably, the leads 12and the die flag 14 have a uniform thickness, wherein the inductivesegments 22 have a thickness sufficiently less than the leads 12 and dieflag 14 to allow the bottom portion of the inductive segments 22 toavoid contact with a substrate or PCB to which the bottom of the leads12 and die flag 14 will contact.

Turning now to FIG. 3, the leadframe 10 of FIG. 2 is illustrated ashaving a semiconductor die 16 and select wirebonds 20 to illustratecertain connections from the semiconductor die 16 to the leads 12, aswell as connections to the inductive segments 22 to form an inductor.Notably, only select bond pads 18 and wirebonds 20 are shown forclarity. Further, each of the inductive segments 22 is furtherreferenced as either inductive segment 22(A), 22(B), or 22(C) forclarity. In addition to the normal wirebond connections between bondpads 18 and leads 12, two wirebonds 20 are shown connecting a bond pad18 to the inductive segment 22(A). Three wirebonds 20 connect inductivesegment 22(B) to 22(C), and another three wirebonds 20 connect inductivesegment 22(A) to 22(C). Multiple wirebonds 20 may be used in parallel tofacilitate higher current flow and minimize resistive losses associatedwith the wirebonds 20. Assume that the lead 12 labeled VCC is intendedto couple to a supply voltage wherein current from the supply voltagemust travel through an inductor prior to reaching the semiconductor die16. Accordingly, the current path is illustrated as traveling from theVCC lead 12 along inductive segment 22(B), over to inductive segment22(C) via wirebonds 20, along inductive segment 22(C), over to inductivesegment 22(A) via wirebonds 20, partially across inductive segment22(A), and over to the semiconductor die 16 via wirebonds 20. In thisfashion, a large inductor can be implemented in the leadframe 10 usingexisting leadframe material and providing a strategic leadframe patternand wirebond connections.

Those skilled in the art should note that an inductor may be implementedusing only one inductive segment 22, and that the example illustrated isprovided only to show a more complicated example and the use ofwirebonds 20 to facilitate interconnection between inductive segments 22and between inductive segments 22 and semiconductor die 16. Further, aninductor may be connected between bond pads 18 of the semiconductor die16 without connecting to a lead 12. Also, the leadframe 10 may bedesigned to provide an inductor between leads 12 without having anyinteraction with the semiconductor die 16, such that the leadframe 10provides an isolated inductive element for use by other circuitryoutside of the given semiconductor die 16.

Turning now to FIG. 4, a partial cross-section of a completesemiconductor and a corresponding portion of a printed circuit boardupon which the semiconductor will mount is illustrated. As shown, theinductive segments 22(A)-22(C) are not as thick as the correspondinglead 12 and die flag 14. A molding compound 26 is used to encase all ofthe elements of the semiconductor, while leaving only the bottomsurfaces of the lead 12 and die flag 14 exposed to facilitate contact tothe printed circuit board 24. In particular, contact is made toconductive traces 28 on the top surface of the PCB 24. The PCB 24 mayalso have conductive traces 30 along the bottom surface and vias 32connecting the top and bottom traces 28, 30.

The molding compound 26 may serve to isolate the inductive segments22(A)-22(C) from the conductive traces 28, as well as hold the inductivesegments 22(A)-22(C), leads 12, die flag 14, semiconductor die 16, andwirebonds 20 in place. Preferably, the lead 12 may be formed with anundercut region to enhance structural integrity and allow the moldingcompound 26 to set in a way that forms a better mechanical connection tothe lead 12. Notably, portions of the leadframe 10, including theinductive segments 22, may extend outside of the semiconductor moldingcompound 26.

FIGS. 5-7 illustrate three exemplary inductive segment configurationswithin a leadframe 10. Preferably, the leadframe 10 is formed usingtraditional etching or stamping techniques to form the leads 12, dieflag 14, and inductive segments 22. The material forming the parts ofthe leadframe 10 may vary depending on application or fabricationtechniques. In the preferred embodiment, the leadframe 10 is formed ofcopper plated with nickel, which is subsequently plated with silver.Those skilled in the art will recognize that the leadframe 10 may beformed using various combinations of platings, materials, layers, andsections. Etching will use lithography and chemical etching to form theleadframe 10, wherein stamping will implement a tool to press and/or cutthe leadframe 10 into the desired pattern and shape.

As noted, the actual inductor formed using the inductive segments 22 mayincorporate all or a portion of any one inductive segment 22, or all ora portion of multiple inductive segments 22. Preferably, the inductivesegments 22 are sized to provide substantially greater inductance thanthe wirebonds 20, and therefore, minimize the impact of the actualwirebonds 20 on the overall inductance provided by the inductivesegments 22. During design and manufacturing processes, the value of agiven inductor will vary based on the length, cross-sectional area, andshape of the inductive segments 22. The length of the inductive segments22 used to form the inductor may be controlled by the selectivepositioning of the point of contact for the wirebonds 20. The points ofconnection for the wirebonds 20 to the inductive segments 22 may befurther adjusted to effectively fine tune the inductance value of theinductor formed by the inductive segments 22. Accordingly, the inductivesegments 22 are the inductive platform for forming inductors based onthe electrical connections, which may be formed using wirebonds 20 orthe actual leads 12. As illustrated above, inductors may be formedacross multiple inductive segments 22 or within a single inductivesegment 22. The inductors formed using the inductive segments 22 haveproven to be substantially less lossy than inductors formed usingwirebonds 20. Further, the inductive segments 22 can form the basis forsignificantly higher inductance values than were previously achievableusing wirebonds 20. In certain applications, the inductors provideinductance value sufficiently high to minimize or eliminate the impactof the inductance in the wirebonds 20.

With reference to FIG. 8, the leadframe 10 may be configured to providemultiple die flags 14A, 14B for multiple semiconductor die 16A, 16B. Theleadframe 10 may incorporate various inductive segments 22, which may beused to form one or more inductors for use in association with thesemiconductor die 16A, 16B or other circuitry.

The present invention provides for integrating inductors into aleadframe in a cost-effective and low-loss manner. The inductance valuesfor the integrated inductors can be programmed within a given rangebased on the design of the leadframe 10 and the points of contact of thewirebonds 20. The maximum inductance for an inductive segment 22 isdetermined by the area available for the leadframe traces forming theinductive segments 22, wherein the final inductance value is selected ortuned by controlling the point of contact for the wirebonds 20. Further,within a given leadframe design, multiple electrical designs andcircuits may be implemented wherein the required inductance is “dialedin” by controlling the position of the wirebonds 20. The programmabilityof the inductance values by controlling the wirebonds 20 reducesmanufacturing and design times. The integration of large inductors intothe leadframe 10 reduces semiconductor fabrication cost, as well asfinal assembly cost, due to the reduced component count and decreasingsize due to integration. The present invention has value in analog,digital, and radio frequency applications.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

1. A method for forming an inductor in a leadframe of a semiconductorpackage comprising: a) forming a leadframe comprising a die flag, leads,and an inductive segment; b) mounting a semiconductor die on the dieflag; c) coupling the semiconductor die to a first point of theinductive segment using a wirebond; and d) coupling a second point ofthe inductive segment to one of the leads or the semiconductor die,wherein the inductive segment is less thick than the die flag and theleads such that a bottom surface of the inductive segment is on a higherplane than bottom surfaces of the die flag and the leads and at leastpart of an inductor is formed between the first and second points of theinductive segment.